The present invention relates to the control of DC motors. In particular, the present invention relates to a system and method for controlling multiple DC motors driving cooling fans.
Much electronic equipment is cooled by fans to remove heat generated by the operation of the equipment. Typically, the fans are incorporated into the chassis (referred to as a fan bank, fan panel, etc.), frame, or cabinet housing the equipment and force air through the cabinet to cool the components contained therein. For equipment that generates a lot of heat, multiple fans are used, and may be switched on or off in response to temperatures inside the frame. It is not uncommon to find a bank of six to eight fans cooling a frame. Each fan may be independently controlled by an associated temperature sensor, or a single temperature sensor may control all fans in the bank simultaneously.
Another arrangement of fan control is to vary the speed of the fans in response to the temperature sensor. Drive circuitry for variable speed fan motors, however, is typically bulky and itself generates heat that has to be removed from the frame by the airflow. Because the direct current (“DC”) motors may draw substantial currents, the drive circuitry typically include, among other electronic components, a large power transistor and an associated heat sink, and a diode.
In conventional fan configurations, the drive circuitry and heat sink for the fan are mounted to the motor hub which tends to impede the airflow through the fan. For efficient airflow, the drive circuitry is required to occupy as small a volume in the hub as possible. This in turn limits the maximum power rating of the drive circuit transistors that can be used, thus limiting the power of the DC motor resulting in low efficiency airflow. For example, for 40 mm and 80 mm motors used in typical cooling fans, about a third of the volume of the motor hub is occupied by drive circuit electronics. In the prior art there are, therefore, limiting factors in the performance of the fan due to the volume occupied by the drive circuitry. An especially critical application, for example, is in rack-mounted systems such as server farms, network switches, etc., where each piece of rack-mounted equipment is restricted in height affording very little room for proper airflow.
One industry standard defines a unit of measure, called a “rack unit,” for a piece of equipment as being ˜44 mm (1.75 inches) in height; referred to as 1 U. The equipment can be integral multiples of 1 U in height, referred to as 2 U components, and so on. In typical rack-mounted systems, the height of the rack can be 42 U, meaning there can be 42 pieces equipment, 1 U tall, mounted into that rack. Heat generation and removal are a significant concern in such densely populated racks.
Another consideration when the drive circuitry is mounted on the motor hub is the cost of replacing a failed motor. As is known in the art, the motor may fail due to overheating of the windings or a mechanical breakdown, e.g., a damaged ball bearing assembly retaining the spindle of the motor. Because the drive circuitry is mounted on the motor hub, it is replaced along with the failed motor. Typically, the most expensive part of the fan is a microcontroller integrated circuit in the drive circuit and not the DC motor itself. Therefore, also taking the other electronic components in the drive circuit into consideration, it is much more costly to replace the motor hub assembly than it is to replace only the DC motor component.
Further, in the prior art, each of the fans operates independently of the other fans and sets its own rotational speed. There is no coordinated control of the set of fans in order to stabilize the airflow through the frame. The feedback that is established between the motor speeds and the reactions of the temperature sensor to the varying airflow caused by the independent motor speeds is not as predictable as a stable airflow caused by coordinated motor speeds.